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Tech transformation: Opaque windows with a switch flick

An innovative process from Harvard researchers lets you alter the opacity of glass with the flick of a switch. And that's just one effort in an increasingly crowded field.

Courtesy of David Clarke/Harvard SEAS

With an applied voltage, the nanowires on either side of the glass become attracted to each other and move toward each other, squeezing and deforming the soft elastomer. Because the nanowires are scattered unevenly across the surface, the elastomer deforms unevenly, causing light to scatter and turning the glass opaque.

Blinds, curtains, shades, drapes, shutters… They could all be relegated to the history books if new window technology developed by Harvard University scientists gains traction.

A new technique, described in the latest issue of Optics Letters, allows the opacity of a window to be controlled by the mere flip of a switch, rendering the glass clear, cloudy, or anywhere in between, all in the space of a second.

While “tunable” windows have been around for a while, past efforts have relied upon electrochemical reactions, expensive to manufacture. The new method employs wiring and electric charge.

“Because this is a physical phenomenon rather than based on a chemical reaction, it is a simpler and potentially cheaper way to achieve commercial tunable windows,” said co-author David Clarke, the Extended Tarr Family Professor of Materials at the Harvard John A. Paulson School of Engineering and Applied Sciences, in a press release.

Here's how it works: a pane of glass or plastic sits between layers of transparent, soft elastomers (elastic polymers, rubbery materials capable of recovering their original shape after being stretched).

Sprayed onto these elastomers are coatings of silver nanowires, too small to interfere with the passage of light on their own.

But, send a pulse of electricity through the nanowires, and they squirm and strain, aching to move closer to one another. In so doing, they squeeze and deform the soft elastomer.

Uneven distribution of the nanowires results in uneven deformation of the elastomer, disrupting the surface, replacing smoothness with roughness, and causing light to scatter, shrouding the once-transparent glass in a cloudy opacity.

The result, said co-author Samuel Shian, is much like a pond in winter.

“If the frozen pond is smooth, you can see through the ice. But if the ice is heavily scratched, you can't see through,” explained Dr. Shian.

The researchers found that the amount of voltage passed through the wires influences the level of opacity achieved: the higher the voltage, the more opaque the glass.

They contrast their work with chemical-based controllable windows, which use “vacuum deposition” to coat the glass, depositing layers of a material molecule by molecule – an expensive and painstaking task.

Some existing technology accomplishes similar results. Last year, for example, researchers at the University of Cincinnati and their industry partners developed a device based on electrodes and the application of voltage that could either be integrated into new windows or easily applied to existing ones.

This research, published in Applied Optics, allowed adjustment not only of the opacity of the glass, but also the color temperature of the light passing through, and indeed the amount of light allowed to cross over to the other side.